The present invention relates to a gear shift system to be used in an automatic transmission for a vehicle.
Both a plurality of planetary gear sets constructing the gear train of the automatic transmission for a vehicle, and frictional engage elements such as clutches or brakes for connecting or fixing the rotating elements of the planetary gear sets selectively are generally accommodated in an integral casing. Moreover, a speed meter gear and a bearing for an output shaft may be accommodated in an extension housing connected to an end portion of the casing, as disclosed in Japanese Patent Laid-Open No. 126174/1993, for example.
This construction will be briefly described in the following. The casing is formed at its rear end portion (as located at the opposite side to the torque converter) with a wall portion which is projected to the radial center. There are provided a multi-disc brake and a one-way clutch for selectively fixing the ring gears of planetary gear sets arranged close to that wall portion. The multi-disc brake has a brake hub made of the ring gear and a brake drum made of the casing such that the brake disc is splined to the outer circumference of the ring gear whereas the brake plate is splined to the inner circumference of the casing. On the other hand, the one-way clutch is arranged between the aforementioned planetary gear set and the wall portion such that its inner race is connected to the aforementioned ring gear and its outer race splined to the inner circumference of the casing. Moreover, the piston for applying the aforementioned multi-disc brake is reciprocally fitted in the aforementioned wall portion. As a result, the piston is formed into a cylindrical shape covering the aforementioned one-way clutch and has its leading end portion formed with a projection which is projected toward the multi-disc brake through a portion of the outer race of the one-way clutch.
As a result, in the gear shift system of the prior art described above, the rotation of the ring gear in a predetermined direction is blocked by applying the one-way clutch, and the piston is moved forward to apply the multi-disc brake to connect and fix the ring gear to the casing. Thus, the reaction at the time of blocking the rotation of the ring gear is received by the casing.
In the gear shift system of the prior art, the piston for applying the multi-disc brake is arranged across the one-way clutch so that it has to be extended toward the multi-disc brake around the one-way clutch. As a result, the casing is radially so enlarged as to cause a disadvantage that the clearance between it and the body is decreased. Since the piston is given the cylindrical shape, there arises another disadvantage that its rigidity has to be sacrificed if its accommodation in the casing is to be improved whereas its size has to be enlarged if a sufficient rigidity is to be ensured. Since, moreover, the piston is formed with the projection extending through a portion of the outer race of the one-way clutch for pushing the brake disc and the brake plate, it is difficult to retain the rigidity at its projection and to push the brake disc and the brake plate completely uniformly around their circumferences. This results in still another disadvantage that the torque capacity may fall short or that an offset wear may occur.
On the other hand, the planetary gear sets are always fed with lubricating oil. Since the aforementioned gear shift system of the prior art is arranged therearound with the multi-disc brake, this multi-disc brake is always fed with much lubricating oil. Moreover, this multi-disc brake is released at the 4th forward speed which is frequently used. Thus, the dragging torque due to the much lubricating oil may be excessively increased to invite a further disadvantage in the reduction of the power loss or in the improvement of mileage. The aforementioned excessive supply of the lubricating oil to the multi-disc brake is also caused by the fact that the piston has a cylindrical shape covering around the one-way clutch, such that the lubricating oil at the inner circumference of the piston is splashed therealong toward the multi-disc brake by the centrifugal force. As a result, the lubricating oil to the multi-disc brake is oversupplied. Still moreover, the structure of the prior art described above has to be made by such complicated works in the deepest portions of the casing that the piston is fitted in the wall portion formed at the axial end portion of the casing, and that the one-way clutch is then fitted while threading the projection of the piston, as desired. Thus, the gear shift system of the prior art is inferior in its assembling works.
On the other hand, generally speaking, the automatic transmission using the planetary gear sets can provide the more gear stages if the number of planetary gear sets and the number of frictional engagement elements for connecting the components of the gear sets selectively grow the larger. However, the increase in the numbers of the planetary gear sets and the frictional engage elements will cause a disadvantage that the total length of the automatic transmission is enlarged. In the prior art, therefore, the frictional engage elements for connecting the components of the existing planetary gear sets selectively are arranged around the existing planetary gear sets. The automatic transmission of this construction can increase the number of gear stages to be set. However, the planetary gear sets have their external diameters restricted so that their gear ratios (i.e., the ratios of the tooth number of the sun gears to the tooth numbers of the ring gears) may not be made higher. As a result, a specific automatic transmission having an overdrive planetary gear set and underdrive planetary gear sets has to bring the overdrive planetary gear set to an overdrive state when a reverse stage is to be set, so that it has a problem of having its shift controls complicated.